This invention relates to a dispersion compensating transmission line and system, and more specifically to a dispersion compensating optical transmission line and system capable of long haul transmission of wavelength-division-multiplexed (WDM) signal light.
In a long haul optical amplification transmission system, dispersion compensation fibers are disposed at appropriate intervals in order to control an accumulated chromatic dispersion within a predetermined value (cf. Japanese Patent Laid-open publication No. Heisei 6-11620 or U.S. Pat. No. 5,361,319).
In wavelength-division-multiplexing optical transmission widely noticed as a method for increasing a transmission capacity, since a chromatic dispersion of an optical transmission fiber differs according to respective wavelengths (dispersion slope), an accumulated chromatic dispersion of each wavelength becomes also different. Firstly, proposed was a configuration that compensated differences of accumulative chromatic dispersion values among the wavelengths at a receiving or transmitting side. However, the dispersion amount practically compensated at the transmitting or receiving side is quite limited. Furthermore, an allowable difference of the dispersion values tends to decrease as a transmission rate per wavelength increases.
Then secondly proposed was an optical transmission system wherein each optical repeating span is composed of a single mode optical fiber having a zero dispersion wavelength at a 1.3 xcexcm band and a slope compensating dispersion compensation fiber for compensating both of its chromatic dispersion and dispersion slope; namely a difference of accumulative chromatic dispersions among wavelengths (cf. op. cit. Japanese Patent Laid-open publication No. Heisei 6-11620 or U.S. Pat. No. 5,361,319 and D. Le Guen et al., xe2x80x9cNarrow Band 640 Gbit/s Soliton WDM transmission over 1200 km of Standard Fibre with 100 km 21 dB Amplifier Spansxe2x80x9d, ECOC ""98, September 1998, Postdeadline papers, pp. 61-63).
FIGS. 2(A) and 2(B) show an optical transmission line for compensating an accumulated chromatic dispersion according to a cycle of several repeating spans and a distance variation of the accumulated chromatic dispersion. A dispersion slope compensating dispersion compensation fiber for compensating the dispersion slope at the same time is employed in the embodiment. FIG. 2(A) shows the configuration of the transmission line and FIG. 2(B) shows the distance variation of the accumulated chromatic dispersion corresponding to the transmission line shown in FIG. 2(A). Reference numeral 10 denotes an optical transmitter for outputting signal light and reference numeral 12 denotes an optical transmission fiber comprising a single mode optical fiber (a dispersion shifted fiber) having a zero dispersion wavelength at a 1.5 xcexcm band. Reference numerals 14 and 16 respectively denote an optical repeating amplifier and a slope compensating dispersion compensation fiber (SCDCF) for reducing accumulated chromatic dispersion values of respective wavelengths xcexl-xcexn into a predetermined value. The dispersion compensation fibers 16 are disposed at a plurality of the optical repeating intervals. The slope compensating dispersion compensation fiber 16 comprises, for instance, a fiber in which polarities of its chromatic dispersion value and dispersion slope are both reverse to those of the optical transmission fiber 12.
Davg indicates a desired chromatic dispersion value of the whole system. A desired value of dispersion compensation by each dispersion compensation fiber 16 is derived by multiplying the desired value Davg by a transmission distance z from a starting point. Dlocal shows a chromatic dispersion value before the dispersion compensation by the dispersion compensation fiber 16, namely the chromatic dispersion value of the optical transmission fiber 12. The accumulated chromatic dispersion increases at the coefficient Dlocal according to the transmission distance. The dispersion compensation fiber 16 reduces, namely compensates the accumulated chromatic dispersion of each wavelength into a value obtained by multiplying Davg by the transmission distance z. Dlocal generally varies according to a wavelength.
In long haul transmission, since nonlinear effect exists to no small extent, an average chromatic dispersion value Davg of a whole system is generally set low other than zero in order to balance the nonlinear effect with the chromatic dispersion value.
FIGS. 3(A) and 3(B) show a conventional transmission line for compensating an accumulated chromatic dispersion at an optical amplification repeating cycle, and a distance variation of the accumulated chromatic dispersion respectively. FIG. 3(A) shows the transmission line and FIG. 3(B) shows the distance variation of the accumulated chromatic dispersion on the transmission line shown in FIG. 3(A). Reference numerals 20 and 22 respectively denote an optical transmitter for outputting signal light and an optical amplification repeater, reference numeral 24 denotes an optical transmission fiber composed of a single mode optical fiber having a zero dispersion wavelength at a 1.3 xcexcm band, and reference numeral 26 denotes a slope compensating dispersion compensation fiber (SCDCF). The optical transmission fiber 24 and slope compensating dispersion compensation fiber (SCDCF) 26 are inserted in each optical repeating span formed by the optical amplification repeater 22. That is, the dispersion compensating cycle is equal to the optical amplification repeating cycle.
A nonlinearity of an optical fiber is generally expressed as n2/Aeff. The reference symbols n2 and Aeff denote a nonlinear constant and an effective core area respectively. The nonlinearity n2/Aeff of a SCDCF is larger than that of a standard single mode optical fiber. In a conventional system that the dispersion compensation fibers 26 are inserted at frequent intervals, the nonlinear effect, which affects the transmission characteristics, becomes larger. In order to perform the long haul transmission while balancing the nonlinear effect with the chromatic dispersion value, the chromatic dispersion value Davg after the dispersion compensation by the dispersion compensation fiber 26, namely the chromatic dispersion value of the whole system should be set relatively high.
As already mentioned above, because the nonlinear effect exists to no small extent in long haul transmission such as transoceanic transmission, the average chromatic dispersion value Davg of the whole system preferably should be a low value other than zero for balancing the nonlinear effect with the chromatic dispersion value.
In the conventional system shown in FIGS. 2(A) and 2(B), the dispersion-shifted fiber is employed as the optical transmission fiber. The chromatic dispersion value of the dispersion-shifted fiber is low at the 1.5 xcexcm band and therefore the influence due to the nonlinearity becomes relatively too large. To put it concretely, in the WDM transmission, owing to the lowness of the local chromatic dispersion value at the interval before the dispersion compensation by the dispersion compensation fiber 16, each interaction length among the respective wavelengths becomes too long causing the large influence of cross phase modulation (XPM), which makes the stable long haul transmission impossible.
On the other hand, when a single mode fiber, which chromatic dispersion value is high at the 1.5 xcexcm band, is employed as the optical transmission fiber 12, each interaction length among the respective wavelengths of the WDM signal light is shortened and thus the influence of the XPM is also suppressed. However, in order to control the accumulated chromatic dispersion value (the absolute value) within a predetermined value, the dispersion compensation fibers 14 should be inserted at shorter intervals. In other words, the dispersion compensating cycle should be shorter and consequently this configuration becomes more similar to that shown in FIG. 3(A).
In the conventional system shown in FIGS. 3(A) and 3(B), on account of employing the single mode optical fiber, which has the zero dispersion wavelength at the 1.3 xcexcm band, as the optical transmission fiber 24, the local chromatic dispersion value becomes high. Accordingly, each interaction length among the respective wavelengths of the WDM signal light is shortened and thus the influence of the XPM is also suppressed. However, due to the frequent insertion of the dispersion compensation fibers 26, the nonlinear effect of the whole system grows large. When such a large nonlinear effect is balanced with the chromatic dispersion value, the chromatic dispersion value Davg after the dispersion compensation becomes excessively high. As a result, the large chromatic dispersion value, conversely, becomes a problem and makes the transmission characteristics deteriorated. Specifically, the troubles most likely to be occurred are jitter and dispersion endurance. These troubles are most serious in long haul transmission such as transoceanic transmission.
An object of the present invention is to provide an optical dispersion compensating transmission line and system for transmitting WDM signal light steadily over a long haul.
According to the present invention, at least one first dispersion compensator disposed at a first dispersion compensation cycle compensates an accumulated chromatic dispersion of signal light so that an average chromatic dispersion is equal to a first desired value and a plurality of second dispersion compensators disposed at a second dispersion compensating cycle shorter than the first dispersion compensating cycle compensate the accumulated chromatic dispersion of the signal light so that an average chromatic dispersion is equal to a second desired value which absolute value is larger than that of the first desired value.
By this configuration, satisfactory transmission characteristics can be obtained within the first dispersion compensating cycle as well as the control of the chromatic dispersion of the whole transmission system can be simplified.